Hepatitis C virus (HCV) infection is a major public health concern affecting approximately 3% of the worid's population (1). HCV persists in up to 80% of people infected (2), and chronic infection can lead to progressive liver disease including cirrhosis and hepatocellular carcinoma. HCV-related liver disease is the single leading indication for liver transplantation throughout the worid. Although new therapies have improved the rates of sustained response, a large proportion of patients fails to respond to antiviral treatment or develop significant drug toxicity, thus remaining at risk for disease progression. A subset of individuals is able to mount effective anti-HCV immune responses, and compelling data underscore the importance of the quality and nature of the CTL response in mediating spontaneous and treatment-induced clearance of HCV. An understanding of HCV-host interactions is required to combat this virus and to develop improved therapies. We propose the following inter-related aims: Specific Aim 1: To comprehensively investigate the role ofthe T cell immunoglobulin and mucin domain-containing protein-3 (Tim-3)/galectin-9 pathway in conferring protective immunity versus persistence in acute and chronic HCV infection. Specific Aim 2: To investigate the effect of manipulating multiple costimulatory/coinhibitory pathways that regulate immune exhaustion and failure of HCV-specific CTLs, define the extent of polyfunctional restoration, and identify the distinct intracellular mechanisms. Specific Aim 3: Determine the phenotypic and fimctional aspects of natural killer T (NKT) cells that can be manipulated in order to enhance anti-HCV efficacy.